feat: Support extending the system prompt (#1167)

crates/goose/src/agents/agent.rs

@@ -28,4 +28,7 @@ pub trait Agent: Send + Sync {
 
     /// Get the total usage of the agent
     async fn usage(&self) -> Vec<ProviderUsage>;
+
+    /// Add custom text to be included in the system prompt
+    async fn extend_system_prompt(&mut self, extension: String);
 }

crates/goose/src/agents/capabilities.rs

@@ -30,6 +30,7 @@ pub struct Capabilities {
     resource_capable_extensions: HashSet<String>,
     provider: Box<dyn Provider>,
     provider_usage: Mutex<Vec<ProviderUsage>>,
+    system_prompt_extensions: Vec<String>,
 }
 
 /// A flattened representation of a resource used by the agent to prepare inference
@@ -88,6 +89,7 @@ impl Capabilities {
             resource_capable_extensions: HashSet::new(),
             provider,
             provider_usage: Mutex::new(Vec::new()),
+            system_prompt_extensions: Vec::new(),
         }
     }
 
@@ -164,6 +166,11 @@ impl Capabilities {
         Ok(())
     }
 
+    /// Add a system prompt extension
+    pub fn add_system_prompt_extension(&mut self, extension: String) {
+        self.system_prompt_extensions.push(extension);
+    }
+
     /// Get a reference to the provider
     pub fn provider(&self) -> &dyn Provider {
         &*self.provider
@@ -303,7 +310,17 @@ impl Capabilities {
         context.insert("extensions", serde_json::to_value(extensions_info).unwrap());
         context.insert("current_date_time", Value::String(current_date_time));
 
-        load_prompt_file("system.md", &context).expect("Prompt should render")
+        let base_prompt = load_prompt_file("system.md", &context).expect("Prompt should render");
+
+        if self.system_prompt_extensions.is_empty() {
+            base_prompt
+        } else {
+            format!(
+                "{}\n\n# Additional Instructions:\n\n{}",
+                base_prompt,
+                self.system_prompt_extensions.join("\n\n")
+            )
+        }
     }
 
     /// Find and return a reference to the appropriate client for a tool call

crates/goose/src/agents/reference.rs

@@ -184,6 +184,11 @@ impl Agent for ReferenceAgent {
         let capabilities = self.capabilities.lock().await;
         capabilities.get_usage().await
     }
+
+    async fn extend_system_prompt(&mut self, extension: String) {
+        let mut capabilities = self.capabilities.lock().await;
+        capabilities.add_system_prompt_extension(extension);
+    }
 }
 
 register_agent!("reference", ReferenceAgent);

crates/goose/src/agents/truncate.rs

@@ -292,6 +292,11 @@ impl Agent for TruncateAgent {
         let capabilities = self.capabilities.lock().await;
         capabilities.get_usage().await
     }
+
+    async fn extend_system_prompt(&mut self, extension: String) {
+        let mut capabilities = self.capabilities.lock().await;
+        capabilities.add_system_prompt_extension(extension);
+    }
 }
 
 register_agent!("truncate", TruncateAgent);